The present invention relates to an apparatus as defined in the preamble of claim 1, and a method as defined in the preamble of claim 7, i.e. an apparatus and method for detecting a short circuit to chassis ground when driving piezoelectric elements.
Piezoelectric elements can be used as actuators because, as is known, they possess the property of contracting or expanding as a function of a voltage applied thereto or occurring therein.
The practical implementation of actuators using piezoelectric elements proves to be advantageous in particular if the actuator in question must perform rapid and/or frequent movements.
The use of piezoelectric elements as actuators proves to be advantageous, inter alia, in fuel injection nozzles for internal combustion engines. Reference is made, for example, to EP 0 371 469 B1 and to EP 0 379 182 B1 regarding the usability of piezoelectric elements in fuel injection nozzles.
Piezoelectric elements are capacitative elements which, as already partially alluded to above, contract and expand in accordance with the particular charge state or the voltage occurring therein or applied thereto. In the example of a fuel injection nozzle, expansion and contraction of piezoelectric elements is used to control valves that manipulate the linear strokes of injection needles. The use of piezoelectric elements with double acting, double seat valves to control corresponding injection needles in a fuel injection system is shown in German patent applications DE 197 42 073 A1 and DE 197 29 844 A1, which are incorporated herein in their entirety.
Fuel injection systems using piezoelectric elements are characterized by the fact that, to a first approximation, piezoelectric elements exhibit a proportional relationship between applied voltage and the linear expansion. In a fuel injection nozzle, for example, implemented as a double acting, double seat valve to control the linear stroke of a needle for fuel injection into a cylinder of an internal combustion engine, the amount of fuel injected into a corresponding cylinder is a function of the time the valve is open, and in the case of the use of a piezoelectric element, the activation voltage applied to the piezoelectric element.
FIG. 6 is a schematic representation of a fuel injection system using a piezoelectric element 2010 as an actuator. Referring to FIG. 6, the piezoelectric element 2010 is electrically energized to expand and contract in response to a given activation voltage. The piezoelectric element 2010 is coupled to a piston 2015. In the expanded state, the piezoelectric element 2010 causes the piston 2015 to protrude into a hydraulic adapter 2020 which contains a hydraulic fluid, for example fuel. As a result of the piezoelectric element""s expansion, a double acting control valve 2025 is hydraulically pushed away from hydraulic adapter 2020 and the valve plug 2035 is extended away from a first closed position 2040. The combination of double acting control valve 2025 and hollow bore 2050 is often referred to as double acting, double seat valve for the reason that when piezoelectric element 2010 is in an unexcited state, the double acting control valve 2025 rests in its first closed position 2040. On the other hand, when the piezoelectric element 2010 is fully extended, it rests in its second closed position 2030. The later position of valve plug 2035 is schematically represented with ghost lines in FIG. 6.
The fuel injection system comprises an injection needle 2070 allowing for injection of fuel from a pressurized fuel supply line 2060 into the cylinder (not shown). When the piezoelectric element 2010 is unexcited or when it is fully extended, the double acting control valve 2025 rests respectively in its first closed position 2040 or in its second closed position 2030. In either case, the hydraulic rail pressure maintains injection needle 2070 at a closed position. Thus, the fuel mixture does not enter into the cylinder (not shown). Conversely, when the piezoelectric element 2010 is excited such that double acting control valve 2025 is in the so-called mid-position with respect to the hollow bore 2050, then there is a pressure drop in the pressurized fuel supply line 2060. This pressure drop results in a pressure differential in the pressurized fuel supply line 2060 between the top and the bottom of the injection needle 2070 so that the injection needle 2070 is lifted allowing for fuel injection into the cylinder (not shown).
It is important to determine and apply an activation voltage with sufficient precision such that, for example, a corresponding valve plug is accurately positioned at the appropriate time in the fuel injection cycle. Thus it is important to be able to detect various problems in the electrical circuit driving the piezoelectric elements. One such problem is a short circuit to chassis ground within, or at the terminals of, one or more of the piezoelectric elements.
It is therefore an object of the present invention to develop the apparatus as defined in the preamble of claim 1 and the method as defined in the preamble of claim 14 to reliably detect a short circuit to chassis ground within, or at the terminals of, one or more of the piezoelectric elements.
This object is achieved, according to the present invention, by way of the features claimed in the characterizing portion of claim 1 (apparatus) and in the characterizing portion of claim 14 (method).
These provide:
the fuel injection system comprises a current diagnosis unit capable of detecting a fault of a current flowing in the driving circuitry within 10 xcexcsec of the fault occurring; and
a current flowing in the driving circuitry is checked in such a manner that a possible fault of the current flowing in the driving circuitry is detected within 10 xcexcsec of the fault occurring.
A short circuit to ground may have different undesirable effects depending on the location of the short with respect to the piezoelectric element and the piezoelectric element driving circuitry. A short circuit at the positive terminal of the piezoelectric element, e.g. used as an actuator, will prevent it from charging. A short at the positive terminal of one piezoelectric element could also prevent the charging of others that are arranged in parallel with it.
A short to ground at the negative terminal of a piezoelectric element could cause the piezoelectric element to be improperly charged when that actuator has not been selected for charging. For example, in the preferred embodiment of the present invention the selector switch for charging a particular actuator is connected in series with the negative terminal of the piezoelectric element. Shorting out that selector switch would cause the to piezoelectric element be continually charged, even when another piezoelectric element has been selected for charging. A possible consequence of such unplanned charging is the unintentional injection of fuel; a situation which is extremely undesirable.
As a result of a short circuit from a piezoelectric element to chassis ground, electrical current will be diverted from portions of piezoelectric element driving circuitry. However, electrical current will continue to flow in other parts of the piezoelectric element driving circuitry where current would normally be expected to flow.
The present invention examines current flow in different parts of the piezoelectric element driving circuitry to detect a short circuit to chassis ground. The present invention detects a short circuit within the piezoelectric element charging and discharging cycle when current would normally be expected to be flowing through particular branches in the piezoelectric element driving circuitry. A short circuit, however, would cause current to be diverted from one of the branches. The abnormal disparity in the current in the two locations is detected by the present invention and an error signal indicating a short circuit is generated.
For example, during the charging cycle when the charging switch is closed current flows through both, a shunt in the voltage source buffer circuit and through a shunt connected in series at the negative terminal of the piezoelectric element. During that cycle, a current detecting circuit is in place to see whether the expected current is flowing in both locations. If current is flowing in the voltage supply buffer shunt resistor, but not the piezoelectric branch shunt resistor, a short circuit is detected and an error message is generated. To detect whether current is flowing normally at both locations in the circuit, a current signal from measuring points corresponding to the respective shunts is received by a comparator circuit. The comparator circuit outputs a signal to a logic circuit representing the difference between the current flows at the two shunts. If the difference in the two current flows is greater than a predetermined maximum then the logic circuit generates an appropriate error signal.
Advantageous developments of the present invention are evident from the dependent claims, the description below, and the figures.